It has been known that there is a risk of explosion in an extremely broad range when a hydrogen gas exists in the air in an amount of 4.0 to 75.0% (% by volume). Accordingly, it is important to measure the hydrogen gas concentration in a low concentration at the lower explosion limit of 4.0% or less. Heretofore, as a gas sensor, there has been known a catalytic combustion type hydrogen gas-detection sensor (see Patent Document 1) in which the temperature of a catalyst such as tin oxide and Pt is raised by a heater and the catalytic action thereof is combined.
Also, as a semiconductor gas sensor, there has been known a sensor in which change in electric resistance is used by utilizing change in carrier density at the surface of the semiconductor due to adsorption of a reducing gas.
In addition, there was a sensor which has been heightened gas selectivity by utilizing absorption or permeation of a specified gas such as hydrogen. For example, as a device to detecting hydrogen by utilizing a hydrogen storage alloy, there has been known a hydrogen-detecting device (see Patent Document 2) which detects a hydrogen-absorption amount based on the size of the detected strain in which the hydrogen storage alloy is adhered to one surface of a substrate, and a strain gage is attached to the other surface, and the strain of the substrate caused by volume expansion of the hydrogen storage alloy when it absorbs the hydrogen is detected by the strain gage.
It has also been proposed a hydrogen detecting device (see Patent Document 3) for detecting a concentration of a hydrogen gas contained in a gas by utilizing a hydrogen storage alloy having high selectivity of hydrogen and detecting change in the state (weight change) when the hydrogen is absorbed while maintaining the hydrogen storage alloy to a constant temperature.
It has heretofore been known as an oxygen gas sensor that an oxygen concentration can be measured by making an oxygen (O) being absorbed (stored) (intercalation) to the interlaminar of titanium disulfide (TiS2) which is a layered crystal and from the change in resistance at that time. In general, it has also been known that by spontaneously absorbing an atom (an oxygen atom in case of an oxygen gas) which constitutes a gas molecule, a temperature of the substance which absorbs the atom increases.
Heretofore, as a temperature sensor, there are an absolute temperature sensor which can measure the absolute temperature and a temperature difference sensor which can measure the temperature difference alone. As the absolute temperature sensor which can measure the absolute temperature, there are a thermistor, a transistor thermistor (Patent Document 4, JP Patent No. 3366590) which uses a transistor as a thermistor and a diode thermistor (Patent Document 5, JP Patent No. 3583704) which uses a diode as a thermistor, which are invented by the present applicant, and further an IC temperature sensor in which the temperature is in a linear relationship with a forward voltage of a diode or a voltage between emitter bases of a transistor. Moreover, as the temperature difference sensor which can measure the temperature difference alone, there have been a thermocouple and a thermopile in which the thermocouples are connected in series to increase output voltage.
It has heretofore been proposed a hydrogen sensor (Patent Document 6) which is constituted by a microcapsule means to encapsulate powder particles of a hydrogen storage alloy with a metal film, a temperature detecting means by a thermocouple, an integrating means in which the powder of the hydrogen storage alloy encapsulated by the microcapsule means and the thermocouple as the temperature detecting means are contained in a cap and an electronic controlling means by an electronic controlling portion including a power source as main characteristics.
The present inventor has also invented previously “a gas sensor element and a gas concentration measurement device using the same” (see Patent Document 7) and proposed a gas sensor element and a gas concentration measurement device which are intended to measure the concentration of a hydrogen gas in which one or a plural number of temperature sensors and a gas-absorbing substance which absorbs a gas to be detected are provided to a thin film thermally separated from a substrate, and the temperature sensors are so provided that temperature change accompanied by heat absorption or heat generation at the time of absorption or release of the gas to be detected. After that, he has conducted experiments and improvement thereof, and as a result, the best embodiments which conform to the various objects can be obtained as the present invention.